Field of the Invention and Related Art Statement
This invention relates to a photomagnetic recording apparatus of a magnetic field modulating system which can make over writing on a photomagnetic recording medium.
Recently, there has been practiced an optical recording/reproducing apparatus which can record and reproduce information at a high recording density.
In the above mentioned optical recording/reproducing apparatus, a recording/reproducing apparatus of a photomagnetic system utilizing a photomagnetic phenomenon has a merit that the written information can be erased and again written in and therefore can be used instead of a recording/reproducing apparatus of a magnetic system using such magnetic head as a hard disc.
In the recording system utilizing a photomagnetism, a magnetic field modulating system of such principle as is shown in FIG. 1 is known as an over writing system simultaneously making erasing and recording.
In FIG. 1, the reference numeral 1 represents a photomagnetic disc. In this prior art example, the photomagnetic disc 1 is formed by coating a resin substrate 2 with a vertically magnetized film 3 and is rotated with the axis L as a center. The reference numeral 6 represents an optical head section radiating a laser light to the vertically magnetized film 3 of the photomagnetic disc. This optical head section 6 is Provided with a semiconductor laser device 4 and focusing lens 5 and can be moved in the radial direction of the photomagnetic disc 1 while keeping a predetermined distance from the surface of the photomagnetic disc 1. An electromagnet 7 is placed in a position opposed to this optical head section 6 through the photomagnetic disc 1 so as to be able to apply a magnetic field to the vertically magnetized film 3 of the photomagnetic disc 1 and can be moved in the radial direction of the photomagnetic disc 1 as operatively connected with the optical head section 6. There is also provided a magnetic field modulating circuit 8 for modulating with recorded data the magnetic field generated by the electromagnet 7 so that, for example, in response to "1" or "0" of the recording signal, the direction of the electric current flowing through the electromagnet may be changed to be in the direction of the arrow A or B and the direction for the photomagnetic disc 1 of the generated magnetic field may be varied.
In the photomagnetic recording apparatus of the thus formed prior art example, as shown, for example, in FIG. 2A, when a recording signal is input into the magnetic field modulating circuit 8, such magnetic field as is shown in FIG. 2B will be generated in the electromagnet 7 and will be applied to the vertically magnetized film 3 of the photomagnetic disc 1. As a result of radiating a laser light of a fixed intensity P which can make the temperature of the vertically magnetized film 3 of the photomagnetic disc 1 above the Curie point as shown in FIG. 2C, such recording pattern as is shown in FIG. 2D will be typically formed in the recording track of the vertically magnetized film 3 of the photomagnetic disc 1. In this FIG. 2D, the marks (+) and (-) represent respectively the states magnetized upwardly and downwardly with respect to the vertically magnetized film surface. Thus, in the photomagnetic recording apparatus of the prior art example, while the minute part of the vertically magnetized film 3 of the photomagnetic disc 1 is being continuously heated to be above the Curie point, the rotary axis L is moved to the center, the magnetic field modulated in response to the recording signal is applied to magnetize the part having become near the Curie point and, when the temperature falls, the magnetization will be held to make photomagnetic recording. According to this system, recording can be made by over writing new informatiion on already recorded magnetized information.
In the photomagnetic recording apparatus adopting the magnetic field modulating system of the above mentioned prior art example, in case the magnetic field applied to the vertically magnetized film 3 of the photomagnetic disc 1 turns its direction in response to the variation of the signal level of the recording signal, such turning time td as is shown in FIG. 2B will be required. When the value of td is large, during this turning time td, the magnetic field of an intensity .vertline..+-.Hc.vertline. required to orient the magnification in the magnetic field direction will not be applied to the vertically magnetized film 3 but a laser light of a fixed intensity P will be continuously radiated to it and therefore, on the recording track pattern corresponding to the magnetic field turning time td, as shown by the reference symbol a in FIG. 2D, a region in which the direction of the magnetic field is not defined will be made and a noise region likely to cause an error at the time of reproduction will be generated. Here, the turning time td is such that, when the inductance of the magnetic head coil is represented by L, the electric current flowing through the coil is represented by I and the voltage applied to the coil at both ends is represented by V, if the coil is assumed to be made of only a pure inductance component, the relation of td=LI/V will be established.
On the other hand, if the number of coils of the head is represented by N, the generated magnetic field Ho of the head will be proportional to NI (Ho.infin.NI) and the inductance L of the head will be proportional to N.sub.2 (L.infin.N.sup.2). Therefore, in order to apply a fixed magnetic field to the photomagnetic disc 1 apart by a fixed distance Sp from the magnetic head, as the number N of coils and current I relating to Ho can not be made smaller, the product LI will be constant. In order to reduce the turning time td while securing the fixed magnetic field Ho, the applied voltage V will be made larger and there will be a problem that the power consumption of the magnetic field driving circuit and magnetic head coil will increase. Also, in the case of connecting a resistance in series with the coil, if the resistance value of the resistance is represented by R, td will be given by td=L/R. Therefore, in this case, too, in order to make td smaller, R will have to be made larger and there will be a problem that the power consumption in the resistance will increase.
Particularly, in the case of a high density recording, it will be necessary to make the turning time td further smaller. If the applied voltage V is made larger for that purpose, the power consumption will further increase and the heat generation by the loss in the magnetic head coil and the driving circuit driving the head will become so large as to make the high density recording difficult.
For example, when the heat generation in the above mentioned magnetic head coil becomes larger, the insulating film of the magnetic head coil will be thermally broken to short-circuit, the magnetic characteristic of the core of the magnetic head will deteriorate, the heat will raise the temperature of the photomagnetic disc 1 and a stable recording will be no longer be made. That is to say, in the magnetic field modulating system, as the direction of the magnetic field varies in response to the recording signal, after the beam spot is irradiated, the magnetization of the magnetized film will have to be quickly defined. For this purpose, after the beam spot is irradiated, the irradiated part of the spot will have to be quickly cooled and therefore, if there is a factor of raising the temperature of the photomagnetic disc 1, no stable recording will be able to be made.
Therefore, as disclosed in Japanese Patent Application Laid Open No. 37842/1988, there is conceived a method of modulating a laser light by applying a magnetic field at a fixed frequency under a resonant condition. In this method, as a magnetic field of a fixed frequency is applied under a resonant condition, the current flowing through the magnetic head winding will be determined by inductance L of the magnetic head and the turning time td will become smaller but, due to the resonant circuit, the modulating signal over a wide frequency range will not be able to be recorded and, after all, the light will be modulated as synchronized with the magnetic field variation. Therefore, in this system, a light regulating circuit is necessary, an applying timing of the light and magnetic field and a controlling mechanism for combining the light spot position on the disc and the magnetic field applying region are necessary and there has been a problem that the formation is more complicated than of a magnetic field modulating system of a simple formation.
On the other hand, the prior art example shown in FIG. 3 is of a system using a floating type magnetic head 11. In this system, a magnetic head 11 is made small and is integral with a slider 12 so that the magnetic head 11 together with the slider 12 may be floated up several .mu.m above the disc 1 by utilizing the pressure by the air stream flowing below the slider 12 when the photomagnetic disc 1 is rotated and the magnetic field may be thereby applied to the photomagnetic disc 1 to record the magnetic field modulation. According to this system, as the distance between the magnetic head 11 and disc is so short as to be several .mu.m, the intensity of the magnetic field generated from the magnetic field generating coil can be made small, the magnetic field region applied to the disc can be also made narrow, therefore the exciting current and inductance can be made smaller than that of a magnetic head of a non-floating type (mentioned as a fixed type) and therefore there is an advantage that the high density recording turning the magnetic field at a high speed becomes easy.
However, in this system, as the distance between the magnetic head 11 and photomagnetic disc 1 is so short as to be several .mu.m, there is a problem that, when the magnetic head 11 and disc 1 contact with each other, the disc 1 surface and head 11 will be damaged or the head 11 and disc 1 will be attracted to each other by moisture or the like and the advantage of the non-contact recording which is a feature of the light recording will be impaired. Also, there are problems that, in order that the magnetic head 11 may be floated above the disc 1 and may be maintained at a height of several .mu.m, the shape of the slider 12 of the head 11 must be worked so precisely as to be adapted to floating up, a supporting mechanism for stably supporting the slider 12 against the air pressure is necessary and the floating type head 11 is more complicated in the structure and higher in the cost than the fixed type head.
Also, the Prior art example shown in FIG. 4 and disclosed in Japanese Patent Application Laid Open No. 37842/1988 is of a system of making recording with a fundamental light modulating system wherein a resonant circuit is formed of an inductor of a magnetic head winding and a externally fitted condenser, a fixed frequency magnetic field is generated from the magnetic head under a resonant condition and a light pulse modulated in response to recorded data by the direction of a magnetic field applied onto a disc is radiated to the disc as synchronized with the magnetic field variation.
In this system, a condenser 15 is connected in series with a coil 14A of an electromagnet 14 to form a resonant circuit 16 and an oscillating circuit 17 having the coil 14A of the electromagnet 14 as an oscillating coil is provided.
In this case, the value C of the condenser 15 is set so that the resonant frequency f0 of this resonant circuit 16 may be a frequency 2 fm twice as high as the highest frequency fm of the recording signal and a magnetic field in which the direction is turned at the frequency 2 fm is generated by the electromagnet 14.
The oscillating signal obtained by an oscillating circuit 17 is fed to one input end of a coinciding circuit 19 through a waveform shaping circuit 18 and the recording signal input into the other record input end is fed to a light modulator 20.
In this system, as the exciting current of the magnetic head is flowed under a resonant condition, even if the inductance of the head is large, a large current of a high frequency will be able to be given to the head winding and, as a result, even in a fixed type head in which the magnetic head is separated by more than 100 .mu.m from the disc, a magnetic field large enough to magnetize a medium turning at a high speed as shown in FIG. 5A will be able to be generated and, when such modulated laser pulse as is shown in FIG. 5B is radiated by the direction, such magnetizing pattern as is shown in FIG. 5C will be able to be formed and recorded on the vertically magnetized film 3 of the disc 1.
According to this system, as the light modulation is recorded while the magnetic field is being varied at a high speed by using a magnetic head fixed as separated from a disc, over writing which has been impossible with an ordinary light modulating system giving a fixed magnetic field is possible and the problems of the floating type magnetic head can be solved. In this system, in addition to the mechanism of varying the magnetic field, a light modulating circuit combining the variation of the magnetic field and timing is required and therefore there is a problem that the structure is more complicated than of the general magnetic field modulating system.